THE RENAL EXCRETION OF FOLIC ACID*

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Abstract
The renal excretion of tritiated folic acid has been explored by using the single renal intra-arterial injection in the dog. Renal tubular reabsorption of folic acid was demonstrated. The initial reabsorptive mechanism from tubular lumen to cell exhibited saturation. It was inhibited by methotrexate. The reabsorbed folic acid was displaced into the urine from renal tubular cells by a large dose of unlabeled folic acid, demonstrating storage of folic acid in these cells under these experimental circumstances rather than rapid transport into the blood stream. The characteristics of this re-absorptive mechanism (saturation, inhibition, and counter transport) are compatible with a mobile, membrane, carrier-transport system for folic acid abutting upon the tubular lumen.